Phytoplankton strategies and diversity under different nutrient levels and planktivorous fish densities in a shallow Mediterranean lake

Two mesocosm experiments were carried out to investigate thedynamic ef fects of nutrients (nitrogen and phosphorus) andplanktivorous fish additions on phytoplankton strategies anddiversity. The phylogenetic and functional approaches were usedto understand phytoplankton ecology in shallow Mediterraneanlakes. The experimental approach is new for the study of algalfunctional groups. Nutrient loading and fish stocks enhancedbiomass of small algae but decreased phytoplankton diversityand species richness. Faster species replacement and fluctuationsin diversity occurred above loadings of 1 µM P and 21µM N. Mesotrophic conditions favoured a diverse pool ofspecies, including nostocales and unicellular flagellate algae(functional groups S_n, S₁, L₀, Y, Reynolds et al., 2002). C-strategistchlorophytes (small algae from functional group X₁) dominatedmid-successional assemblages with good light and accessiblenutrients. High nutrient concentrations, dim light, presenceof organic matter and of larger zooplankton favoured to functionalgroups S₁ of oligophotic filamentous cyanobacteria and J ofmixotrophic Scenedemus species. Intermediate nutrient levelswith total phosphorus (TP) 10 µM, water quiescence, transparencyand smaller zooplankton prompted dominance of chroococcal cyanobacteria(functional groups X₁ and K). Resulting patterns agree and reinforcethe validity of plankton functional groups associated with warm,shallow enriched systems, although some changes in the groupsare suggested in relation to the structuring role of nutrientsand grazing on the functional scheme for phytoplankton.     

Uptake of 13C and 15N (ammonium, nitrate and urea) by Microcystis in Lake Kasumigaura

The uptake rate of carbon and nitrogen (ammonium, nitrate andurea) by the Microcystis predominating among phytoplankton wasinvestigated in the summer of 1984 in Takahamaira Bay of LakeKasumigaura. The V_max values of Microcystis for nitrate (0.025–0.046h^–1) and ammonium (0.15–0.17 h^–1) were considerablyhigher than other natural phytoplankton. The ammonium, nitrateand urea uptake by Microcystis was light dependent and was notinhibited with nigh light intensity. The K₁ values were farlower than the I_k values. The carbon uptake was not influencedby nitrogen enrichment. Microcystis accelerated the uptake rateby changing V_max/K _s value when nitrogen versus carbon contentin cells declined. Nitrate was scarcely existent in TakahamairiBay during the summer, when Microcystis usually used ammoniumas the nitrogen source. However, the standing stock of ammoniumin the water was far lower than the daily ammonium uptake rates.Therefore, the ammonium in this water had to be supplied becauseof its rapid turn-over time (–0.7–2.6 h).     

Role of light, carbon dioxide and nitrogen in regulation of buoyancy, growth and bloom formation of Anabaena flos-aquae

The availability of light, CO₂ and NH₄-N interacted to controlbuoyancy and growth of the gas vacuolate blue-green alga, Anabaenaflos-aquae. At high light intensities algal growth rates werehigh; however, the alga was non-buoyant regardless of the availabilityof CO₂ or NH₄-N. The mechanism for buoyancy loss involved increasedcell turgor pressures at higher light intensities which resultedin collapse of gas vacuoles. At lower light intensities algalgrowth rates and cell turgor pressures were reduced and buoyancywas controlled by the availability of CO₂ and inorganic nitrogen.Carbon dioxide limitation increased buoyancy, while reducedinorganic nitrogen availability reduced buoyancy. Mechanismsfor buoyancy regulation at low light intensities involved changesin cellular C/N ratios which appeared to affect the rate ofsynthesis and accumulation of protein-rich gas vacuoles. Algalspecific growth rates were combined with buoyancy data to forma single index (µ_bloom) to the rate of surface bloom formationof A.flos-aquae as a function of the availability of light,CO₂ and NH₄-N. The bloom formation index was enhanced with decreasedavailability of light and CO₂, and increased availability ofNH₄-N.     

Diurnal mixed layers and the long-term dominance of Microcystis aeruginosa

The population dynamics of the cyanobacterium Microcystis aeruginosain a hypertrophic, subtropical lake (Hartbeespoort Dam, SouthAfrica) were followed over 4.5 years. We examined the hypothesisthat M.aeruginosa dominated (>80% by volume) the phytoplanktonpopulation up to 10 months of each year because it maintaineditself within shallow diurnal mixed layers and was thus ensuredaccess to light, the major limiting resource. Wind speeds overHartbeespoort Dam were strong enough to mix the entire epilimnionthrough Langmuir circulations only 12% of the time. At othertimes solar heating led to the formation of diurnal mixed layers(z₁) that were shallower (<2 m) than the euphotic zone (z_cu;mean = 3.5 m, range: 0.45–8.4 m) while the seasonal mixedlayer (z_m) was always deeper than z_cu (range: 7–18 m).By means of its buoyancy mechanism M.aeruginosa maintained thebulk of its population within z₁, while non-buoyant speciessank into dark layers. Adaptation to strong light intensitywas implicated from low cellular chlorophyll a content (0.132µg/106 cells) and high I_k (up to 1230 µE m^–2s^–1). Ensured access to light, the post-maximum summerpopulations persisted throughout autumn and winter, despitesuboptimal temperatures, by sustaining low losses. Increasedsedimentation losses caused a sharp decline of the populationat the end of winter each year, and a short (2–3 months)successional episode followed, but by late spring M.aeruginosawas again dominant. The data from Hartbeespoort Dam point outthe importance of distinguishing between z_m and z₁, and showthe profound effect that the daily pattern of z₁ as opposedto the seasonal pattern of z_m, can have on phytoplankton populationspecies composition.     

Pigment ingestion and egg production rates of the calanoid copepod Temora longicornisr. implications for gut pigment loss and omnivorous feeding

We examined the relationship between egg production rate (E)and pigment ingestion rate (I, from gut content corrected for33% loss) for adult female Temora longicornis in Long IslandSound on 47 occasions. Linear regression of E on I [both variablesexpressed in mass of nitrogen (N) female^–1 day^–1]was: E_N = 0.0016 + 0.770 x I_N. The slope, 0.77, is the apparentgross efficiency of egg production, equivalent to the grossgrowth efficiency (GGE) assuming that females partition allnitrogen for growth into egg production. Published work suggeststhat a GGE of 0.37 would be expected for herbivorous copepods.The discrepancy between the expected value of 0.37 and observedvalue of 0.77 could result from unquantified losses of gut pigmentor because T.longicomis ingested a significant amount of nitrogenby feeding as a carnivore. We suggest that if T.longicomis femalesderive all of their nitrogen for growth by feeding on phytoplankton,and if no correction for pigment loss is employed, then thegut pigment method underestimates pigment ingestion by no morethan a factor of two.     

Photoinhibition and the diurnal variation of phytoplankton photosynthesis--I. Development of a photosynthesis--irradiance model from studies of in situ responses

Diurnal series of fluorescence and photosynthesis assays wereconducted in high altitude (3803 m), tropical (16°), LakeTiticaca (Peru/Bolivia). Near-surface diurnal thermoclines formedon typical days of high photon flux density (PFD, {small tilde}2000 µE m^–2 s^–1). In the depth range of diurnalstratification profiles of in vivo fluorescence, both without(F_a and with (F_b DCMU, exhibited a mean decrease of 64% frommorning to mid-day, but little change (mean increase of 1.5%)through the afternoon. Three times during the day surface, mid-depth(3–5 m) and deep (15–20 m) phytoplankton sampleswere incubated with H¹⁴CO₃^– under short (<2 h) exposuresto a range of in situ PFDs. Comparison of phytoplankton in differentsamples (ANOVA) showed identical photosynthetic response insunrise (isothermal) samples but a significant drop in surfaceand mid-depth photosynthesis at all PFDs during times of diurnalstratification. Similarly, both low-light () and light-saturated(P² _max photosynthetic parameters were lower in mid-day surfacesamples compared to deep samples. In addition, previously photoinhibitedsamples had a higher threshold intensity for photoinhibition,I_T. These results, together with diurnal time series of fluorescencefrom in situ incubations, demonstrate that recovery from extendedepisodes of photoinhibition during diurnal stratification isslower than suggested by previous observations in vitro. Photosynthesisby near-surface phytoplankton is different in light increasingup to I_T than light decreasing from I_T. This effect can be modeledby reducing and P_max as a function of the maximum photoinhibitingPFD in the diurnal light history. ¹Present address: Division of Molecular Plant Biology, Universityof California, Berkeley, Berkeley, CA 94720, USA     

The seasonal productivity of phytoplankton in a hypertrophic, gravel-pit lake

The seasonal time course of phytoplankton primary productivitywas studied weekly in a hypertrophic, gravel-pit lake closeto Madrid, Spain. Chlorophyll a ranged 22–445 mg m^–2.Gross primary productivity attained 0.28±0.14 g C m^–2h^–1 (range: 0.06–0.60), its yearly value being 900g C m^–2, but the shallow euphotic depths and the highplankton respiration ensured that net productivity was generallylow. Respiration losses amounted to 0.31±0.24 g O₂ m^–2h^–1, with phytoplankton respiration roughly attainingone-half of overall plankton respiration. Areal phytoplanktonproductivity and plankton respiration followed a seasonal trendbut this was not the case for photosynthetic capacity. Surfacephotoinhibition was evenly distributed throughout the study.Quantum yields showed an increasing depth trend, but no seasonaltrend. Both P_max and I_k were both temperature- and irradiance-dependent.As compared with lakes of lesser trophic degree, phytoplanktonprimary production in hypertrophic lakes might be increasednot only by higher nutrient contents but also by low chlorophyll-specificattenuation coefficients and low background, non-algal attenuation,thereby allowing for higher areal chlorophyll contents and hencehigher areal productivity. Our study suggests that physical(irradiance and water column stability) as well as chemicalfeatures (dissolved inorganic carbon and soluble reactive phosphorus)may control seasonality of phytoplankton primary productionin this lake despite recent claims that only physical factorsare of significance in hypertrophic lakes. However, this doesnot explain all the variability observed and so a food web controlis also likely to be operating.     

Strong spatial differentiation of N and P deficiency,primary productivity and community composition between Nyanza Gulf and Lake Victoria (Kenya,East Africa) and the implications for nutrient management

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Sustained membrane depolarization and pulmonary artery smooth muscle cell proliferation

Pulmonary vasoconstriction and vascularmedial hypertrophy greatly contribute to the elevated pulmonaryvascular resistance in patients with pulmonary hypertension. A rise incytosolic free Ca²⁺ ([Ca²⁺]_cyt)in pulmonary artery smooth muscle cells (PASMC) triggers vasoconstriction and stimulates cell growth. Membrane potential (E_m) regulates[Ca²⁺]_cyt by governing Ca²⁺influx through voltage-dependent Ca²⁺ channels. Thusintracellular Ca²⁺ may serve as a shared signaltransduction element that leads to pulmonary vasoconstriction andvascular remodeling. In PASMC, activity of voltage-gated K⁺(Kv) channels regulates resting E_m. In thisstudy, we investigated whether changes of Kv currents[I_K(V)], E_m, and[Ca²⁺]_cyt affect cell growth by comparingthese parameters in proliferating and growth-arrested PASMC. Serumdeprivation induced growth arrest of PASMC, whereas chelation ofextracellular Ca²⁺ abolished PASMC growth. Resting[Ca²⁺]_cyt was significantly higher, andresting E_m was more depolarized, inproliferating PASMC than in growth-arrested cells. Consistently, wholecell I_K(V) was significantly attenuated in PASMCduring proliferation. Furthermore, E_mdepolarization significantly increased resting[Ca²⁺]_cyt and augmented agonist-mediatedrises in [Ca²⁺]_cyt in the absence ofextracellular Ca²⁺. These results demonstrate that reducedI_K(V), depolarized E_m, and elevated [Ca²⁺]_cyt may play a criticalrole in stimulating PASMC proliferation. Pulmonary vascular medialhypertrophy in patients with pulmonary hypertension may be partlycaused by a membrane depolarization-mediated increase in[Ca²⁺]_cyt in PASMC.

     

Light Distribution in Discontinuous Canopies: Calculation of Leaf Areas and Canopy Volumes Above Defined 'Irradiance Contours' for use in Productivity Modelling

Plant canopies can be considered as assemblages of leaves, stemsand fruits growing in zones of differing irradiance demarcatedby contours of mean irradiance as measured on a horizontal surface. The following general equations have been derived to calculatethe leaf area (L_I) and the canopy volume (CV_I) in zones externalto any chosen contour of mean irradiance: (1) L_I = ((1nl)/(–K)(I–T_f) or leaf area index (LAI) if this is less (2) CV_I = L_I/(leaf area density m² m^–2), where I is the specified value of irradiance (horizontal surface)expressed as a decimal fraction of that above the canopy, Kis the appropriate extinction coefficient and T_f is the proportionof the total of available radiation which, if the canopy isdiscontinuous, would reach the ground by passing through gapsbetween the discrete canopy units. Where the canopy is continuousT_f is zero so expression (1) simplifies to L₁ = 1n I/–K(or LAI if this is less). For a range of model hedgerow orchards of varying dimensions,spacings and LAIs, it has been shown that the use of these equationsgives very similar results to those obtained by detailed calculationof light penetration. They therefore seem to be of potentialuse in calculating both potential dry-matter production by discontinuouscanopies of any type and, in the case of orchard fruit crops,the potential effect of changes in tree size, leaf area density,spacing etc. on the canopy volume in which irradiation is adequatefor fruit bud initiation and fruit colour development. light distribution, discontinuous canopy, irradiance contours, leaf area index, orchards     

Spatial and temporal patterns of phytoplankton composition in subtropical coastal lagoon, the Indian River Lagoon, Florida, USA

A 2 year study of the phytoplankton community was carried outin the Indian River Lagoon, USA. In terms of biovolume, thephytoplankton community was generally dominated by dinoflagellates,diatoms or cyanobacteria. Mean phytoplankton standing cropswere highest in the most flow-restricted regions of the lagoon,which had the lowest mean salinity values and comparativelyhigh total nitrogen:total phosphorus ratios. In this region,blooms of dinoflagellates were common in the first year of thestudy, which was characterized by an El Niño event thatyielded exceptionally high rainfall levels and freshwater outflow.Picoplanktonic cyanobacteria blooms became more prominent inthe second year of the study, which was characterized by belowaverage rainfall conditions. In unrestricted flow regions ofthe lagoon, located near inlets to the Atlantic Ocean, diatomswere most often the dominant taxa. Regions of intermediate waterturnover rates and high external loading of phosphorus had aprevalence of diatom blooms. However, the average phytoplanktonstanding crops in the latter regions did not reach the levelsexperienced in the flow-restricted parts of the lagoon. In termsof individual phytoplankton taxa, the most common bloom-formingdiatoms in the Indian River Lagoon system included: Skeletonemacostatum, Dactyliosolen fragilissimus, Skeletonema menzelii,Cerataulina pelagica, Odontella regia, Chaetoceros lorenzianus,Rhizosolenia setigera and Thalassionema nitzschioides. The majorbloom-forming dinoflagellate species included: Pheopolykrikoshartmannii, Akashiwo sanguinea, Prorocentrum micans, the potentiallytoxic species Pyrodinium bahamense var. bahamense and Prorocentrumminimum. Several picoplanktonic cyanobacteria were also prominentmembers of the phytoplankton community, including Synechococcuselongates. The spatial and temporal patterns observed in someof these dominant species were attributable to patterns in keyenvironmental variables, including salinity, temperature andnutrient concentrations.     

Leaf Growth in Cotton (Gossypium hirsutum L.) 2. The Influence of Temperature, Light, Water Stress and Root Restriction on the Growth and Initiation of Leaves

The rate parameters R₁, R₂, I/LI and I/t_0.5, which characterizethe growth in area of successive main-stem leaves, probablyall have the same temperature response. Temperature thereforeonly operates on the time scale. Water stress reduces both therelative growth rate and the advance of developmental age, thelatter however to a lesser extent than the former. The effectof root restriction is explained as resulting from mineral shortage. Gossypium hirsutum L., cotton, leaf growth, leaf initiation, relative growth rate, temperature, light, water stress, root restriction     

Influence of nitrogen on growth and photosynthesis of a C3 cereal, Oryza sativa, infected with the root hemiparasite Striga hermonthica

Striga hermonthica is a root hemiparasitic angiosperm nativeto the African semi-arid tropics. It is a major weed of C₄ cerealsbut locally it is also an important weed of the C₃ plant, rice[Oryza sativa). Infected rice plants produced 17% and 42% ofthe total biomass of uninfected plants when grown at two differentammonium nitrate concentrations, 1 and 3 mol m^–3, respectively.S. hermonthica prevented grain production at both concentrationsof nitrogen. At the lower concentration no heads were produced.At the higher concentration head weight was only 6% of uninfectedcontrols. S. hermonthica also altered the partitioning of drymatter between plant parts, such that shoot growth was reducedto a greater extent than root growth. As a consequence the root-to-shootratio of infected plants was approximately five times greaterthan that of uninfected control plants. Light saturated ratesof photosynthesis In infected plants were 56% and 70% of thoseof uninfected controls, at low and high nitrogen, respectively.Infection also led to lower values of stomatal conductance althoughthe substom-atal CO₂ concentration was unaffected. Analysisof the response of photosynthesis to substomatal CO₂ concentration(A/C_I curves) demonstrated that lower rates of photosynthesiscould not be solely attributed to lower stomatal conductances.Lower initial slopes and asymptotic rates suggest that bothcarboxylation and processes controlling regeneration of ribulose-1,5-bisphosphate are reduced by infection. The data are discussedwith respect to the influence of S. hermonthica on the growthand photosynthesis of C₄ hosts, where in contrast to the situationwith rice, nitrogen feeding results in a marked alleviationof the effects of the parasite on the host. Key words: Rice, Striga, growth, photosynthesis, nitrogen     

Cyanobacterial nitrogen fixation in arid soils of Central Australia

Abstract The distribution of nitrogen-fixing cyanobacteria was studied in two regions of Central Australia, one near Yulara and the other in the Ti-Tree Basin. Soil and other samples were tested in situ for nitrogen fixation by the acethylene reduction assay. They were then enriched in low light in suitable growth medium containing no fixed carbon or nitrogen over a period of months. The cyanobacterial components were identified and their nitrogen-fixing capacity measured again both in the light and in darkness. It is concluded that a variety of genera of cyanobacteria survive in the arid region of Central Australia and can be assumed to contribute fixed nitrogen to the soils of the region.     

An empirical model of primary productivity (14C) using mesocosm data along a nutrient gradient

The two parameters of the hyperbolic tangent equation, P_m and, were estimated from in situ vertical profiles of primary productionusing mesocosm data along a nutrient gradient. The parameters,derived from 4-h (around noon) ¹⁴C incubations, were used togetherwith the photosynthesis-light curve and hourly solar radiationdata to calculate daily primary production rates (P_d). Approximately40% of the daily production occurred in the 4 h around noon.Considering parameter uncertainty, there was no indication ofan increase in variation in production with increased nutrientloading, nor did biomass-specific P-I parameters increase. Annualproduction ranged from 82 to 901 g C m^–2 year^–1and was highest in the highest nutrient treatment tank. Dailyproductivity ranged from 0.02 to 9.1 g C m^–2 day^–1and was significantly correlated, in all treatments, with acomposite parameter BI₀/k (where B is phytoplankton biomass;I₀ is daily radiation and k is the extinction coefficient).Linear regressions of P_d against BI₀/k indicated that much ofthe variability (86%) in productivity was explained by lightavailability and phytoplankton biomass. Two approaches for predictingproductivity were compared: (i) predicting production directlyfrom environmental variables (i.e. BI₀/k) and (ii) predictingthe parameters of the P-I curve from environmental variablesand using these to calculate daily production.     

Nutrient-limited growth rates: quantitative benefits of stress responses and some aspects of regulation

Young sunflower plants (Helianthus annuus L.) under stress oflow nitrate or phosphate availability exhibited increases inroot: shoot ratio and in kinetic parameters for uptake. Theyshowed no significant changes in photosynthetic utilizationof either nutrient. Increases in root: shoot ratio were achievedby early and persistent suppression of shoot growth, but notroot growth. Affinity for phosphate uptake, 1/K_m(P), increasedwith phosphate stress, as did affinity for nitrate uptake, 1/K_m(N),with nitrate stress. Maximal uptake rate, V_max, for phosphateuptake increased with phosphorus stress; V_max for nitrate didnot increase with nitrogen stress. Phosphate V_max was relatedstrongly to root nutrient status. Decreases in V_max with plantage were not well explained by changes in age structure of roots.Estimated benefits of acclimatory changes in root: shoot ratioand uptake kinetics ranged up to 2-fold increases in relativegrowth rate, RGR. The relation of RGR to uptake physiology followedpredictions of functional balance moderately well, with somesystematic deviations. Analyses of RGR using growth models implyno significant growth benefit from regulating Vmax, specifically,not from down-regulating it at high nutrient availability. Quantitativebenefits of increases in root: shoot ratio and uptake parametersare predicted to be quite small under common conditions whereinnutrient concentrations are significantly depleted by uptake.The root: shoot response is estimated to confer the smallestbenefit under non-depleting conditions and the largest benefitunder depleting conditions. Even then, the absolute benefitis predicted to be small, possibly excepting the case of heterogeneoussoils. Depleting and non-depleting conditions are addressedwith very different experimental techniques. We note that atheoretical framework is lacking that spans both these cases,other than purely numerical formulations that are not readilyinterpreted. Key words: Nutrient stress, nutrient uptake, nutrient use efficiency, relative growth rate, Helianthus annuus     

Quantitative Analysis of ATP-Dependent H+ Efflux and Pump Current Driven by an Electrogenic Pump in Nitellopsis obtusa

Internodal cells of Nitellopsis were made tonoplast-free byperfusion with a medium containing EGTA. Cytoplasmic concentrationsof solutes were controlled by a second perfusion with mediaof known composition. The electrogenic pump current (I_p), whichwas calculated from electrical data obtained from cells withand without ATP, was compared with the current carried by H⁺(I_H⁺) across the plasma membrane. A close correlation betweenI_p and I_H⁺ was found under various internal and external conditions.(1) I_p and I_H⁺ depended on the internal ATP and showed Michaelis-Mententype saturation curves. For I_p, K_m was 120 µM and themaximum current V_max was 15.1 mA m^–2, while for I_H⁺, K_mwas 160 µM and V_max was 16.6 mA m^–2. (2) I_p andI_H⁺ showed almost the same I_H²⁺ dependence. The Mg²⁺-dependentI_p was 19.5 mA m^–2, while the Mg²⁺-dependent I_H²⁺ was17.7 mA m^–2. (3) I_H²⁺ was maximal at an external pH of8 and decreased both in acidic and alkaline pH ranges. I_p wasnearly equal to I_H⁺ in the pH range between 8 and 5. (4) I_H⁺became maximal at an internal pH of 7.3, which is nearly thesame as the pH for maximal electrogenecity found by Mimura andTazawa (1984). All these facts support the idea proposed in our previous paper(Takeshige et al. 1985) that the electrogenic ion pump locatedin the plasma membrane of Nitellopsis is the H⁺ pump. ¹ Dedicated to Professor Dr. Erwin Bünning on the occasionof his 80th birthday. (Received June 21, 1985; Accepted December 20, 1985)     

Nitrogen Use Efficiency in Growth of Polygonum cuspidatum Sieb. et Zucc

The growth of Polygonum cuspidatum in sand culture was analysedunder varying nutrient conditions. Nitrogen availability influencednitrogen uptake of plants through the uptake rate per unit rootweight rather than the amount of root. In turn, the differentamounts of nitrogen taken up affected plant growth through theireffects on the rate of leaf expansion. Net assimilation rate (NAR) increased with nitrogen contentper unit leaf area (C), but further increase in leaf nitrogencaused diminishing returns of NAR Optimal nitrogen content perunit leaf area (C_opt) to maximize dry-matter production of aleaf could be determined by drawing a tangent from the onginto a curvilinear relation between NAR and C. This optimal contentdivides a nitrogen-limiting range (C < C_opt) from a carbon-limitingone (C> C_opt) along the axis of nitrogen content. Under nitrogenlimitation, efficiency of nitrogen use in dry-matter productioncould increase if the plant had a larger carbon sink. This givesa qualitative explanation to reduced shoot-to-root ratio underlimited availability of nitrogen. Polygonum cuspidatum Sieb. et Zucc, Japanese knotweed, carbon sink, growth analysis, leaf nitrogen, net assimilation rate, nitrogen use efficiency     

Phytoplankton photosynthesis parameters in central Canadian lakes

Values for two biological parameters — (i) P^B_m, the lightsaturated rate of photosynthesis per unit of chlorophyll and(ii) , the slope of the light limited part of the photosynthesisversus light curve per unit of chlorophyll — must be knownin order to be able to estimate rates of phytoplankton primaryproduction from chlorophyll data. These parameters were measuredfor periods of up to 9 years in central Canadian lakes locatedin temperate, subarctic and arctic climatic zones. Regardlessof their geographic locations, lakes where integral photosynthesiswas nutrient limited had lower values of these parameters thandid lakes where integral photosynthesis was light limited. Temperatureset an upper limit to the variability of P^B_m but was not a goodpredictor of its actual value. Year-to-year variability of photosynthesisparameters in the most intensively studied group of lakes waslarge: annual means varied by a factor of three over a 9-yearperiod. Until the sources and extent of this variability areknown, accurate production estimates can be obtained from chlorophylldata only if P^B_m and are measured in each water body and inevery year. Implications for estimating primary production fromremotely-sensed chlorophyll data are discussed.